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This invention relates generally to implantable electrodes and electronic hybrid circuits and in particular to implantable electrodes and electronic hybrid circuits having a liquid crystal substrate.
Microelectronic components, integrated circuits, and implantable electrodes are used extensively in implantable medical devices (IMDs) such as cardiac pacemakers, cochlear prosthesis devices, and neuroprostheses. IMDs can be constructed using a variety of well known methods such as printed circuit boards and hybrid circuits formed on a substrate. Typical hybrid circuits are used can be formed using well known techniques. As the device size and conductor size decrease to below approximately 10 micrometers, the hybrid substrate must be micro-machined using photolithographic techniques to pattern and put down the conductor traces. An IMD needs to be encased with an encapsulant such as silicone that is chemically bonded to the substrate.
Implanting medical devices in a biological environment subjects the IMD to a chemically and electrically harsh environment. For example, the biological environment is highly corrosive to many materials, and the conductors used to connect the device to other electronic circuits or connectors must be able to withstand immersion in an ionic fluid with as much as a 10-volt bias across it.
Cardiac pacemakers typically include a hermetically sealed titanium canister containing the power source and associated circuitry and glass sealed electrode feed-throughs to allow the electronic signals generated by the circuitry to interface to the heart muscle. The size of cardiac pacemakers is dominated by the size of the energy source, and typically, the titanium case is a few centimeters in diameter and half a centimeter thick. The leads are typically multi-filament coils of a high nickel content stainless steel alloy and the leads are typically insulated with using silicone. Silicone insulated leads have been very reliable, however, silicone has a tendency to stick to tissue during insertion and to reduce the diameter of the pacemaker leads.
IMDs for neuroprostheses have even more demanding requirements than cardiac pacemakers. Neuroprostheses for rehabilitation of the deaf, blind, spinal cord injured and amputees are being developed that make use of IMDs. In these instances, the IMD requires close proximity to the small and fragile cells of the nervous system. In some cases, the IMDs will be attached or embedded directly in the neural tissue. The neural tissue is a very dynamic environment, for example peripheral nerves stretch and relax with the motion of a limb, the spinal cord moves within the spinal canal, the brain moves relative to the skull any motion of the head and also with each heartbeat, and movement of the eyes creates substantial acceleration forces on the retina.
Because of the nature of the biological environment, the fragile nature of the neural tissue, the high packing density of the neural tissue, the effects of dissimilar acceleration, and the proliferation of connective tissue that can encase an IMD, IMDs used as neuroprostheses must be biocompatible, bioresistant, be of small size, be density matched to the surrounding neural tissue and be minimally tethered to the surrounding tissue.
Biocompatibility is essential in an IMD to minimize the formation of connective tissue between the nearby neurons and the IMD over the course of long term or chronic implantation. Bioresistance, or chemical inertness with respect to the biological environment is essential to prevent corrosion from damaging the IMD. An IMD needs a small size to minimize damage to the target neural structures during implantation. To avoid differential acceleration between the IMD and the surrounding tissue, matching the density of the two is important to avoid damage to the surrounding tissue. Minimal tethering between an electrode and an electronic device will reduce the transmission of forces transmitted along the wiring between the electronic device and the implanted electrode, particularly after being encased in connective tissue as part of the normal healing process.
Silicon has been the material of choice for neuroprosthetic IMDs because of its mechanical and chemical properties. For example, silicon can be micro-machined to extraordinarily small dimensions, is very strong, relatively corrosion resistant, can have embedded integrated circuits for signal processing or controlling functions, and because it forms an inert self limiting oxide that is biocompatible. Silicon may be micro-machined to produce a variety of novel structures. Silicones are an important class of materials that can both insulate silicon substrates as well as protect silicon substrates from corrosive environments. However, although silicone has been shown useful as an encapsulant, silicone has not been useful as a micro-machined substrate because it is not dimensionally stable and thus cannot support fine metal patterns or be photolithographically processed.
Of the many candidate materials that have been used in the prior art, only polyimide was a possible polymer that could be used for flexible implantable microelectrode array substrates. Polyimide has been used extensively by researchers for producing microelectrode arrays for cochlear electrode arrays, retinal prostheses, peripheral nerve electrodes, and central nerve electrodes. While polymer based flexible electrodes have been previously developed using polyimide, polyimide is not a very long-term water resistant material. Furthermore, polyimide is used as a sensor for humidity because of its hygroscopic quality. Although polyimide structures may be able to withstand up to several years of static immersion in saline, the failure modes of polymide structures are usually linked to mechanical weakening of the material due to hydrolytic attack.
Micro-machined silicon substrates as fabricated are not bioresistant and can have multiple failure modes when an integrated circuit or microelectronic hybrid circuit are formed thereon. The wires used to attach to the circuit elements must be able to withstand immersion in ionic fluids. Exposed areas where the wires are attached to connectors or devices are coated with encapsulant material that is applied after wire bonding as been accomplished. If micro-ribbon technology is used, it is necessary to create a void free seal in the area under the micro-ribbon attached to the device. Circuits on the chip must be protected from water and ionic contamination and the chip substrate and encapsulants must be bioresistant and biocompatible.
Therefore, it would be advantageous to provide a substrate and/or encapsulant for an IMD that has is biocompatible, bioresistant, small size, and has a density that is matched to the surrounding neural tissue.
An implantable medical device (IMD) is disclosed that is formed on a substrate composed of liquid crystal polymer (LCP). In one embodiment, the IMD can be an interconnection module for interconnecting an electrode array to an equipment module. The interconnecting module includes conductors disposed on the LCP substrate and coupled to the electrode array, and wherein the conductors are encapsulated using a silicone or LCP encapsulant. In another embodiment, the IMD is an electrode array and interconnect module disposed on an integral LCP substrate. An equipment module can be coupled to the interconnect module. Alternatively, a hybrid electronic circuit can be coupled to the interconnect module for signal processing and conditioning signals received from the electrode array or for providing stimulus signals to the electrode array. In this embodiment, all of the conductors and at least a portion of the electrodes in the electrode array are encapsulated using a silicone or LCP encapsulant. In another embodiment, the IMD is an electrode array, an interconnecting module, and a hybrid circuit that are disposed on an LCP substrate. The interconnecting module is used to provide signal paths to and from the electrodes in the electrode array to the hybrid circuit. In this embodiment, all of the conductors, the hybrid electronic circuit and at least a portion of the electrodes in the electrode array are encapsulated using a silicone or LCP encapsulant.
Other forms, features and aspects of the above-described methods and system are described in the detailed description that follows.